1. Field of the Invention
The present invention relates to photography and, more particularly, to photographic apparatus having a gear train for transmitting power from a motor to one or more motor-driven components of the apparatus.
2. Description of the Prior Art
Compact cameras having motor-driven components are well known in the photographic art. For example, highly automated, self-developing cameras, such as the SX-70 Land Camera, marketed by Polaroid Corporation, Cambridge, Mass., include a motor-driven processing roller (one of a pair of pressure-applying rollers), film advance device, camera sequencing or timing wheel, and a device for recocking a reflex operator.
To minimize the size of the camera, the motors are generally of the small, high speed, D.C. type with typical loaded operating speeds of 8,000 to 12,000 rpm when energized by a six-volt battery. Motor power is transferred to each of the motor-driven components by a gear train which serves to (1) provide appropriate speed reductions for driving the various components, and (2) physically couple the motor to components that are mounted at various locations within the camera housing.
The SX-70 camera includes a base housing section having an open ended chamber therein for receiving a film container holding a stack of self-developing film units. Couple to the leading open end of the base section is a loading door and pressure roller mounting section which is adapted to pivot between an operative position; wherein the loading door section is aligned with and extends forwardly of the open end of the receiving chamber to locate the rollers in position to receive a film unit advanced from the container subsequent to exposure; and an inoperative or open position wherein the loading door section is pivoted downwardly to unblock the open end of the receiving chamber for loading and/or withdrawing the film container.
The small high speed motor is located at the trailing end of the base section, aft of the receiving chamber. Power is transmitted to the various motor driven components by an elongated speed reduction gear train that is disposed along the side of the chamber and extends from the motor in the rear to one of the pressure-applying rollers mounted on the forwardly extending loading door section.
Since the gear train spans the interface between the leading end of the base section and the trailing end of the loading door, two meshing gears at the interface must be disengaged to permit the door to be opened and then must be easily and reliably brought back into mesh, without binding, or being brought into nonmeshing abutment, when the loading door section is closed. This type of structure may be thought of as a "hinged gear train" having one gear mounted on the base section and its mating gear mounted on the loading door section so that these two gears mesh and bridge the interface when the loading door section is closed.
When the loading door is closed, these two gears are in mesh. Upon opening the door and disengaging the two gears, the rotational position of the gear at the leading end of the base section is fairly stable because it is coupled all the way back to the de-energized motor which is rendered inoperable by a door switch when the loading door section is opened. On the other hand, the mating gear on the loading door section is coupled to the top pressure roller (in a low friction bearing) and, because of the low gear loading, the mating gear may be thought to be in a "free wheel" condition. The angular or rotational disposition of the mating gear will most likely change (from its position when in mesh) by (1) the act of disengagement, (2) the gear being inadvertently rotated by the user during the process of loading a film container or (3) inspecting and/or performing maintenance on the rollers.
To ensure that these two gears will mesh properly when the loading door is closed, the two gears are identical. That is they have the equal diameters, a speed ratio of 1:1, equal number of gear teeth about the periphery or pitch circles, identical gear tooth profiles and equal tip spacing between adjacent teeth. To minimize possible tip-to-tip abutment when the gears are brought back into mesh, the standard gear blunt tip tooth profile may be modified so that the tips are pointed. This, coupled with the fact that the gear on the loading door is pivoted into engagement with the gear on base section and tends to have a free wheeling rolling action when the two gears make initial contact, greatly facilitates the ease and reliability of bringing these two gears into mesh without causing damage to the gears or inconvenience to the user.
For representative examples of the "hinged gear train" concept outlined above, reference may be had to U.S. Pat. Nos. 3,709,122; 3,714,879; 3,760,701; and 3,906,527 assigned to the same assignee as the present application. Also see U.S. Pat. No. 3,561,340 which discloses a camera having a motor-driven roller assembly (which may be removed for maintenance) that is coupled to a gear train by a pair of 1:1 gears at the point of separation.
The use of 1:1 gears at the point of separation essentially adds a pair of gears to the gear train to facilitate bringing the two sections of the train back into mesh. On the negative side, however, is the question of gear train efficiency or power loss. As the number of gears in a train increases, the power transfer efficiency of the train decreases because of, among other considerations, the friction losses at the meshing teeth and also at the shafts on which the gears are mounted for rotation.
It has been found, that when the 1:1 gears (at the point of separation) have been replaced with a pair of speed reduction or speed increasing gears, in the interest of gear-train efficiency, the problem of getting the two nonidentical gears to remesh properly becomes a serious one.
For example, see U.S. Pat. No. 3,889,280 which discloses a large gear mounted at the leading end of the camera base section which is adapted to mesh with a smaller gear (pinion) coupled to the top roller to provide a speed increase (i.e. the roller rotates at a faster speed than the larger driving gear).
Also, notice should be taken of the following copending applications which feature a pair of speed reduction gears at the point of separation. Ser. No. 554,777, filed on Mar. 3, 1975, by B. K. Johnson et al.; Ser. No. 554,778 (now U.S. Pat. No. 3,967,304), filed on Mar. 3, 1975, by B. K. Johnson et al.; and Ser. No. 628,486, filed on Nov. 4, 1975, by R. M. Augustin et al.; all of said copending applications being assigned to the same assignee as the present application.
When the pair of gears at the point of separation have different numbers of teeth (and correspondingly different diameters to produce speed reduction or increase), the tip spacing or chordal distance between adjacent teeth at the tips will be different (by definitions which will be developed later in this disclosure). If standard gear profile teeth (blunt tips) are used, the nonmeshing problem is even further compounded because of the increased probability of tooth tip abutment.
When attempts have been made to modify standard gears (for example, a 12-tooth pinion and 36-tooth gear) by increasing the tooth length to form points at the tip, it has been found that the tip spacing on the pinion (smaller gear) increases to a larger extent than the tip spacing on the larger gear. In some cases, this may cause an interlock condition when the two gears are attempted to be pivoted into mesh. That is, the opposite outside edges of two adjacent teeth on the large gear may fit into the space subtended by the two inside edges of a pair of adjacent teeth on the pinion.
Also, when one of the gears in a set is a small diameter pinion, it is common practice in gear technology to extend the addendum or length of the pinion teeth to prevent undercut (reduction of the thickness of the tooth at its base) and to decrease the length of the teeth on the mating gear. Again, this promotes tip spacing mismatch and has a tendency to cause the interlock condition.